An Investigation of the Electric Smelting of Copper Concentrates

AN INVESTIGATION OF THE ELECTRIC SMELTING OF COPPER CONCENTRATES

John w. Goth

A thesis presented to the Faculty of Graduate Studies and Research of McGill University in partial fulfilment of the requirements for the degree of Master of Engineering.

McGill University August, 1951 Montreal, Canada ACKNOWLEDGMENTS

I wish to express my appreciation to Professor J. u. MacEwan, the director of the investigation, and to the staff of the Metallurgical Department who aided in as sembling and adapting the equipment for the investigation. I also wish to thank Professor G. s. Sproule for his assistance and helpful advice, and to thank both

Professor MacEwan and Professor Sproule for giving so much of their time to the investigation. The writer greatly appreciates their helpfulness and assistance. TABLE OF CONTENTS

Page

INTRODUCTION •• ...... • • • • • 1 Theoretical and Historical Discussion ...... 2 The Reverberatory Furnaoe • • • • • • • • • • • • • 7 The Blast Furnace • • • • • • • • • • • • • • 8

Copper Deposits and Resouroes ••••••••••• 9

The Electric Furnace. 10 THE SCOPE OF THE INVESTIGATION • ...... 14 THEORETICAL DISCUSSION ...... 15

&~PERIMENTAL PROCEDURE • ...... 20 Apparatus ...... 20

Procedure . . . • • • ...... • • • 24 Smelting Raw Concentrates ••••••••••••• 27 EXPERIMENTAL RESULTS • ...... 28 DISCUSSIOlJ OF RESULTS •• ...... 31 Design of Small Commercial Type Furnace •••••• 33 SUMMARY...... 38 APPENDIX ...... 39 BIBLIOGRAPHY ...... 43 ILLUSTRATIONS

Figure I The free energy relationships between Cu2S and FeS; Cu20 and FeO ••••••••••• 3 II Cu2S•FeS Constitution Diagram ••••••• 5 III Ternary FeO•Si02•CaO Constitution Diagram • 7 IV Schematio drawing of furnaoe used in the investigation • • • • • • • • • • • • • • • 22

V Schematic drawing of small scale commercial type furnace. • • • • • • • • • • • • •

V( a) Seotional views of the above •• ...... 37 VI Picture of the furnace used during the in vestigation showing auxiliary equipment • •

VII Slag showing included matte • • • • • • • •

VIII Matte containing 53.17% copper, showing homo geneous solid solution structure ••••

IX Matte c ontaining 36.24% copper, showing the euteotiferous structure • • • • INTRODUCTION

Matte smelting of eopper ores and eoneentrates in North America

is carried on entirely in reverberatory and blast furnaces; but in

Scandinavia, where fuels are scarce and hydro-electric power is available in quantity, electric furnaces are used for smelting operations. Each type of smelting furnace has its own peculiar advantages and disadvantages~ the furnace used being a matter of economies. The reverberatory furnace with its auxiliary equipment is suited for large soale operations only, as the capital oost of the plant is high and operation must be continuous.

It is operated with pulverized coal, natural gas, or oil for fuels, the selection being a matter of oost. The ore feed must be fine which makes the furnace ideally suited for the treatment of copper concentrates. The blast furnace can be operated over a wide range of capacities, but it is not as economical to operate this type of furnace for large tonnages as it is with the reverberatory. Also the charge must be coarse for easy blast penetration which means an additional high sintering oost for fine ores and concentrates. The only fuel used is coke whieh is mixed in with the charge. The electric furnace can be operated over a wide range of capacities and, in fact, is the only one of the above types that can be operated on a small scale. No fuel is necessary as nearly all the heat is supplied by the current, chemical reactions in the charge making a minor contribution.

There have been two recent developments in reverberatory fur nace practice. In Finland, a reverberatory furnace is being operated - 2 - with no extraneous fuel, the necessary heat for smelting being supplied by the oxidation of the metal sulphides by preheated air, as well as by small quantities from other chemical reactions. The heat in the waste gases is efficiently used to preheat the combustion air. The other development, not yet in complete operation, is the use of oxygen instead of air for the direct smelting of high sulphur concentrates and s~itable fluxes. These operations must be considered on a large scala with a heavy capital investment.

Electric furnace smelting offers a number of attractive features, particularly today with the widespread use and distribution of hydro- electric power. An adequate, econamical power supply in a region where fuel costs are high is advantageous to the electric furnace. This furnace can be operated in relatively small sizes thus offering the possibility for the small isolated operator ta process his concentrates into high grade matte or even to blister copper which can be freighted to the re- fineries at a lower oost.

Theoretical and Historical Discussion

The free energies of the cammon metal sulphides show cuprous sulphide to be much more stable than ferrous sulphide(l9)*. These free energy relationships for copper and iron in the oxide and sulphide form are shawn in Figure I. These two heavy metals are the only ones that will form sulphides in copper smelting. The other metal oxides such as lime and magnesia are tao stable, while others such as nickel, cobalt, and manganese are not present in appreciable quantities.

* References are at the end of the thesis. - 3-

0 _,..., V' ~ - -40 / • v """Œ! ~ / Va / ~ ~ -80 v --- ~ / -120 0 400 800 1200 1600 2000 2400

Temp. degrees c. Standard free energies of formation of metal oxides for reactions involving one gram mole of oxygen.

-10

-20 / . -30 Y" ·~ ~ ~ -40 .,/' / v-- - c:t ~ ~ -50 ~ _... y -60 / /" -70 200 400 600 800 1000 1200 1400 1600 1800

Temp. degrees C. Standard free energies of formation of metal sulphides for reactions involving one gram mole of S02 gas. (19)

Fig. I - 4-

The iron sulphide, copper sulphide constitution diagram, Fig ure II,(2 ) shows their combining characteristics. It appears from this diagram that ferrous sulphide is completely soluble in cuprous sulphide in the liquid state. The melting points of the cuprous sulphide and fer rous sulphide are approximately 1130°0. and 1170°0. The two sulphides are completely soluble in one another in the liquid state, and they form a eutectiferous solid solution upon cooling. This diagram does not include the presence of any other compounds, but the assumption that matte is composed of the above sulphides only is sufficiently accurate for our purposes. Actually matte is a more complex substance than this would indicate.

In high contrast with the sulphides of iron and copper, the free energies of their oxides(l9) show that ferrous oxide is a much more stable compound than cuprous oxide. From the above data calculations show that the equilibrium for the reaction

goes nearly completely to the right. The same reaction occurs when copper reacts with ferrous sulphide.

2Cu + FeS = Cu2S + Fe

This iron is afterwards oxidized by the furnace gases to ferrous oxide which entera the slag. It can be stated that the copper combines nearly campletely with the sulphur to form cuprous sulphide, and the remaining sulphur unites with the iron to form ferrous sulphide. As stated before this is not entirely true. Small amounts of MnS, CoS, ZnS, and NiS will be found in the matte depending on the metal content of the charge.

Magnetite and ferrio oxide formed in roasting and smelting are oammon constituants in the charge to the copper smelting furnaoe. Where - 5 -

Liquid

1200

1100

1000 Cl.) ., 1 1 rz. 1 1 4- .Dim.CU"phi

Fig. II - 6 - there is an intense reducing atmosphere as in coke-fired blast furnaces. the magnetite and terrie oxide are reduced to the ferrous state. In fur naces operated with nearly neutra! atmospheres, this does not occur. How ever, equilibrium studies indicate that these two iron oxides react with ferrous sulphide to give ferrous oxide and sulphur dioxide. This reaction rate is very slow, however, and equilibrium is hardly ever obtained in practice. The reactions are theoretically thus:

3Fe203 + FeS • 7Fe0 + SQ2

3Fe;04 + FeS • lOFeO + SQ2 Any remaining magnetite has a high specifie gravity and is round in vary ing concentrations in the matte.

Satisfactory slags will have a number of definite properties:

(a) low fusion point

(b) high fluidity

(d) low solubility for matte

(e) low specifie gravity

Copper smelting slags are primarily iron silicates with various other oxides to lower the melting points, specifie gravities. and solubility for matte in slag. Thus CaO, MgO, A120;, and ZnO are found in slags.

Slag of the composition 40 per cent FeO. 40 per cent Si02. and 20 per cent of the other oxides assumed to act like lime will be found around the red circle of the diagram shown in Figure III. These slags will have a low melting point, and the high FeO content will give good fluidity at furnace temperatures. The slag does not carry high cost fluxes, and the solubility for matte and the specifie gravitywill be satisfactory. The slag will - 7 -

s,o~

Fig. III

Phase (partial composition) diagram for ternary system CaO•FeO•SiÛ2

Temperature Contours Outline the liquidus surface.

(Bowen, Schairer, and Posnjak){8)

have an approximate silicate degree of 1.35 to 1.50.

The Reverberatory Furnace

In modern practice the more important type of copper smelting

operation is carried on in the reverberatory furnace. This furnace is used for large scale operations only, the modern reverberatory having a capacity up to 1200 tons of solid charge daily. Its minimum capacity is about 650 tons daily, including fluxes. If much smaller units are used, smelting costa rise sharply and it would not be economically feasible to operate such a unit. - 8 -

The main fuel of the reverberatory furnace is either natural gas, fuel oil, or pulverized coal. The quantity of material smelted by the gross B.t.u. of these fuels is about the same. Gas is used in regions where it can be obtained easily. The heat is distributed in the rever beratory by means of the hot combustion gases passing over the charge.

The heat loss of this furnace is considerable because the gases leave the furnace at a temperature of 2200°F. Waste heat boilers are used to re cover approximately half of this heat. The heat balance of a coal-fired reverberatory will show that the furnace has an efficiency of 30 to 40 per cent neglecting the heat recovery from the boilers. This heat re covery will closely equal the amount of heat used in the furnace.

The Blast Furnace

The blast furnace used for copper smelting requires less spaoe and less capital investment than the reverberatory. It is used for small operations only. The charge to the furnace cannot be too fine. If it were, the flue dust loss would become excessive; and the fine charge would tend to obstruct the passages in the shaft of the furnace preventing gas penetration and thereby deoreasing the furnaoe oapaoity and effioienoy.

The only fuel used in the blast furnace is coke which is mixed with the charge producing a reducing atmosphere.

The blast furnace is used for smaller capacities than the rever beratory, but to be econo.mical a minimum charge of 250 tons per day is necessary. The products of the blast furnace are the same as the rever beratory. The recovery of copper is high provided the ores are not con taminated with compounds of zinc and barium which cause irregular operation of the furnace. The blast furnace is now seldom used for copper smelting. - 9 -

The low grade copper ores have necessitated concentration methods which produce a fine charge. Sintering would be necessary for efficient opera tion 0 f the furnace.

Copper Deposits and Resources

Copper deposits,to be economie in addition to being of good grade, must be of considerable extent; sufficient to warrant the erection and operation of a smelter over a number of years to permit a reasonable depletion rate. If the ore deposit is small, it must be located suffi ciently close to a custom smelter so the freight charges are not exces sive. There are undoubtedlymany auch small deposits that are located unfavorably unless some means or change in conditions is brought about to make their operation profitable. The mineral resources of Canada and the United States are adequate, but they are vanishing assets. It will be necessary in the future to provide a more economical means of recovery to develop these resources.

The location of copper deposits is spread throughout Canada and the United States. The main producing areas are located in Quebec,

Ontario, and Manitoba, and in the western section of the United States.

Many more deposits have been discovered, but these deposits have not been developed. This is undoubtedly because of the geographie location of these 9eposits or other factors which tend to discourage exploitation until more economie recovery methods are available.

The use of electrical power for smelting has a favorable pros pect in Canada due to the number of large and widespread hydro-electric developments. The power may be transmitted into regions where the other - 10 - fuels could not be transported econamically. Present facilities for electric power are adequate, but new projects are being planned to in crease the hydro-electric facilities for new and present industries.

In remote regions where hydro-electric facilities are not available, electrioity can be produced by diesel generators.

The use of electricity for smelting copper ores and concentrates will have many advantages. The electric furnace may be constructed and operated efficiently over a wide range of sizes and with a minimum of skilled help because of the high degree of mechanized control. Electrical energy in quantity is available in most areas through transmission lines from local power stations. The outlook for future hydro-electric power development is favorable.

It was with this prospect in mind that the investigation into the mnelting of copper ores and concentrates in the electric furnace was undertaken. If the feasibility of electric smelting is proven and the technique made available, the owners of isolated small copper deposits will be encouraged to open their mines, operate their concentrators, and process the ores into a matte of a good grade, or blister copper.

This would increase the economy of the country and provide an additional source for a metal that is now scarce.

The Electric Furnace

The electric furnace was first conceived in the early part of the 19th century, the principle of electrical heat energy having been discovered as early as 1815. The eleotric arc was discovered in 1800 by

Sir Humphry Davy shortly after the battery was invented; but until the construction of the dynamo, very little progress was made in deve1oping - ll - this new principle. The first furnace of importance was constructed by

Sir W. Siemens in 1878. (3) Since then the growth and development of the electric furnace has been rapid. The carbon arc has come to be one of the more important factors in the use of electricity for heating purposes.

By the end of the 19th century_ many furnaces were being constructed, using either the carbon arc or the material itself as a resistor.

Some of these types include the horizontal arc and the vertical arc using either carbon or water-cooled electrodes. Other furnaces were constructed in which the container itself was an electrode, and the current passed through the charge. The arc furnaces use the electric arc for radiant heat as in the horizontal electrode furnace, or pass the current through the charge as in the vertical electrode furnace. By the turn of the century, the electric furnace was well conceived and had been put into experimental use.

In industry the eleetric furnace was first used to produce cal- cium carbide and other Sllnilar products. With improved techniques, ferro- alloys were produced. France and Sweden both succeeded in adapting the furnace to the production of good quality steel from scrap steel, pig iron, etc.; as well as in producing alloy steels. The actual smelting of iron ores began in 1898 when an Italian, Captain Stassano, patented his furnace and also demonstrated his work. At the time, however_ the oost of electricity was so excessive that Captain Stassano•s discovery was of no practical value when trying to compete against the coke-fired blast furnace.

The electric furnace may be described as an appliance in which materials are heated to a high temperature by the dissipation of electrical - 12 - energy. Heat is produced whenever an electric current encounters a resistance to its flow. The heat evolved is the dissipation of the energy produced by the current and the resistance through which it passes.

There are two types of electric furnaoes, the arc furnace and the resist ance furnace.

The electric arc furnace is so named because the heat is gene rated by the formation of an arc between the electrodes. There are vari ous types of arc furnaces. The single-phase independant arc furnace has the arc produced between two movable electrodes. The heat is transferred to the charge by radiation. The arc or arcs may be generated by either one, two, or three-phase current Most arc furnaces use three-phase current between three electrodes and the charge.

The Heroult three-phase furnace uses the charge as a cammon pole, and the charge is heated directly. The electrodes are placed vertically in the furnace. The resistance of the arcs and the charge to the electric current generates an amount of heat depending on the power. The hearth of this furnace is made conductive so the current may be carried into the charge. This furnace may also be operated by the use of a non-conducting hearth. The current passes from one electrode through the charge to the other electrodes. The furnace voltage will include the potential drop of both of the arcs and will ordinarily be greater than in the single-arc furnace. The single-arc furnace has greater experimental use because it is easy to control the arc and thereby regulate the temperature.

The electrical resistance furnace generates heat by passing the current through either a solid or liquid resistor. This type furnace may also be divided into two classes, one in which a special resistor is - 13 - provided, and one where the charge is the resistor. These furnaces vary considerably in operation and construction. The furnace with a special resistor is the more comnton of our electrical turnaces today. It is used over a wide range of temperatures and sizes, and it can be accurately con trolled mechanically. Its use is extremely diversified, and this furnace may be found in practically every modern industry. The common furnace is that with the resistor imbedded in or hanging on the walls. Current is passed through this resistor to produce a certain temperature below its melting point. The resistance to the current will cause the resistor to beat the furnace by conduction and radiation. The furnace will eventually reach the same temperature as the resistor. The ability to regulate the temperature of this furnace has made it a valuable aid in beat treating operations.

The majority of the electric smelting furnaces are resistance furnaces, the charge being the resistor. The current is passed through the charge in many ways. Various furnaoe designs use horizontal electrodes, vertical electrodes with one pole at the bottom of the furnace, and pairs or trios of vertical electrodes. The current may pass through any number of electrodes depending upon the power supply and the design of the furnaoe.

A three-phase eurrent ean be conducted through the bath by three electrodes, four electrodes, or three pairs of electrodes. Most furnaces are now de signed to pass the current through the charge between pairs of vertical electrodes. This type of furnace is the one used in the investigation.

This furnace can be operated using the charge as the resistor or by using the charge as the resistor and raising the electrodes,to forma small arc at each electrode to supplement the heat provided by the resistance of the - 14- charge to the current. The former method is the one preferred in the in vestigation although at times it would be necessary to forman arc to keep

the charge molten around an electrode.

A cold charge will not conduct a current, but most molten charges have good conductivities. A cold charge may be heated to a molten state by various methods; the one used in this investigation produced the desired resulta. A trough filled with granular carbon was placed on a layer of slag between the two electrodes to conduct the initial current. The resistance of the carbon to the current caused i t to becom.e incandescent and transmit sufficient beat to melt the surrounding slag and fonn a molten bath. When this slag beeomes molten, charging ean begin and continue until there is a complete layer of solid material over the molten bath. The radiant heat of the bath is used to preheat the charge. The eonductivity of the slag is high enough that by using a low voltage it is possible to pass a high current through the bath.

The efficiency of auch a furnace will increase with its aize.

The operation temperature is lower than that produced by the electric arc, and the temperature is very uniform throughout the smelting zone. The volatilization of impurities is also low. Very little loss is incurred by dust escaping in the flue gas. The working conditions are very favor able; the temperature outside will not deviate much from room temperature.

The heat loss through the furnace walls will not be excessive.

THE SCOPE OF THE INVESTIGATION

On searching through books, journals, and transactions for data on an electric smelting furnace, no reference was found which stated a - 15 - procedure or set of data for a copper smelting operation. The purpose of this investigation is to compile a set of data for an electric smelting furnace, and from these data to investigate the feasibility of the electric smelting of copper concentrates.

THEORETICAL DISCUSSION

Copper can be recovered from sulphur free minerals by smelting in a blast furnace using fluxes and coke, but this is rarely practised except for the treatment of scrap. Copper occurs in ores chiefly as sul phide minerals, and matte smelting of copper ores and concentrates is the predominant method of recovery. Sulphur elimination by roasting approxi mates a first order reaction and consequently the elimination of sulphur to a high degree at moderate temperatures becomes a costly operation. The temperature of roasting is kept moderate to prevent fusion on the roasting hearths, and an amount of sulphur is eliminated to give the desired grade of matte.

The determination of the proper grade of matte in a smelting operation is one of the most important factors in the treatment process.

As the copper content of the charge is fixed, the grade of matte is varied by the amount of sulphur available for the matte. At first it may seem best to operate with a high grade matte, but consideration will show that this is seldom the best practice.

At smelting temperatures a given slag has a more or less definite solubility for matte. Therefore a high grade matte will have a greater amount of copper in the slag. The figures in the following table illustrate this fact:(2) - 16 -

TABLE I

Grade of.' Matte Copper in Slag Plant Per Cent Per Cent

Noranda 19.3 0.27 Flin Flon 22.0 0.36 Anaconda ~.0 0.50 Andes 45·9 o.6o Miami 45·9 0.52 Roan Antelope 78.7 1.20

For any given operation sorne particular slag composition is most economical and theref.'ore the daily slag tonnage is f.'ixed. As the oopper content of the matte increases, the copper content of the slag increases and the total loss of copper to the slag increases a proportional amount, making it advisable to operate with mattes of lower grades. Matte is the solvent for gold and silver and their loss varies directly as that of copper ..

As mentioned above, the sulphur elimination becomes increasingly costly as the concentration decreases. Roasting is not a first order reaction but approximates it. The mathematioal relationship is:

de ko --dt = c = concentration at any time

t = t:ime k = constant

The r.ate given in this equation is at constant temperature; the rate changes with changes in temperature. The f.'act that a high temperature cannot be used means that sulphide ores and concentrates will require a lengthy roasting time for high sulphur elimination which is not always praotioal. - 17 -

The ~uel is oonsumed in roasting to a law sulphide content and the extra

time and operation of equipment all indicate that roasting should take

place only when it is necessary to remove excess sulphur under proper

economie roasting conditions.

The precious metals present in a charge are in good practice

dissolved in the matte with a high recovery. A high grade matte whioh has a small volume will not come in contact with the whole of the charge and therefore will not absorb the precious metals to a high degree. The precious metal recovery increases rapidly as the volume of matte in creases. A low grade matte has a large volume to absorb these metals and will therefore yield a higher recovery. The recovery of precious metals is an important factor in the mnelting process as nearly all the copper ores and concentrates, as well as many fluxes, contain silver and gold.

Also the oonverting of low grade matte is a very exothermio re action. The heat released in converting can be used to smelt some quantity of ore and thereby eut the fuel costs proportionately. These factors all indicate that the matte grade must be closely controlled during the smelt ing operation. The optimum grade of matte is dependent on the composition of the charge as well as many other ~actors and must be determined for each particular operation. The composition of most mattes varies ~rom 13 to 60 per cent copper.

The matte is composed o~ copper and iron sulphides plus minor amounts of impurities. Copper combines with sulphur to form a very stable sulphide, Cu2S• The high temperature stable form of copper sulphide is

Cu2S not CuS. Free energy relationships show that Cu2S is much more stable - 18 -

than iron in the sulphide form, FeS. From the data published by K. K.

Kelley (24-28), and a few reasonable assumptions, the free energy equa-

tion can be developed from the following reaction at 1500°K.

2Cu(l) + FeS(l) : Cu2s(l) + Fe(~)

G0 = 3560 - 54· 5T By use of this and the equation

0 G : -RT ln Kp : -4.58T log Kp

log Kp = 11.38

Assuming the activities are not greatly different from the concentrations, and that free iron and copper are present:

log Kp = ~~ = 11.38 ~

Hence practically all of the eopper combines with sulphur as cu2s, the remaining sulphur combining with the iron as FeS. This is assuming that equilibrium is reached.

The melting point of matte will vary with the composition as shown in the constitution diagram page 5· A matte containing 20 per cent

Cu2S, or one containing 75 per cent cu2s, will each melt at 1080°C. Cam- positions between these limits will melt at lower temperatures. The speci fie gravity of mattes will vary from 4.8 to 5.6, increasing with copper content. - 19 -

The composition of the slag will consist of oxides, principally

FeO, CaO, and Si02. It can be shown that oxygen will combine with iron to fonn a more stable compound than the oxide of copper. From ther.mo- dynamic d~ta, it can also be shown that oxygen will combine preferentially with iron rather than with copper to for.m FeO at 1500°K. The free energy equation is developed from the following reaction with some assumptions:

!gain assuming the activities are not greatly different from the concentra- tiens, and that free iron and copper are present; it can be shown that

0 G • -1000 - 15.7T

By use of this and the equation

G0 = -4.58T log Kp log Kp = 3·57

This indicates that the iron will fonn a more stable oxide than will copper. Iron that does not combine with sulphur to form FeS will go to the slag in the form of FeO. Practical slags vary greatly in composition and are very co.mplex. The melting points are therefore difficult to estimate and can only be determined by direct measurement. Slags are usually tapped from the furnace ranging from 1100° to 1300°C.

The specifie gravity of slag will vary from 2.8 to 3.8. This is considerably lower than that of the matte, permitting a good separation of the two constituants if allowed to stand sorne time in the molten state. - 20 -

Slags that have a high CaO and Si02 content have a lower copper content than those high in FeO. However, it seldom pays to add barran fluxes to

t':i l;;.}, ,·;,: give lower copper mattes. These fluxes will increase the slag volume but will not affect the copper recover,y as the oopper loss is the product of tonnage and copper content of the slag.

EXPERIMENTAL PROCEDURE

Apparatus

The type of apparatus used in the investigation is shown in

Figure IV. This furnaoe was originally designed for the smelting of iron ores. but it was rebuilt and adapted to the smelting of copper concentrates.

The furnace was lined with a 4à-inch course of Walsh XX fireclay refractory brick backed by a 2~inch course of HW-26 insulating fire brick. The two courses of brick formed a furnace wall sevan inches thick inside the steel shell. The bricks were of standard size, and no mortar was used, the bricks being fitted together as tightly as possible. The furnace dimensions were too small to cause trouble fran thermal expansion. A matte trap was built at the front of the furnace extending through the walls to the pouring lip.

To conserve heat the walls of the furnace were extended thirteen inches in height with a 4à-inch course of refractory brick supported by an angle-iron framework bolted to the furnace. An arched roof was built over this addition with a charging door in the center. The door was seldom used as the front section of the roof was left open during the investigation to facilitate charging, visual inspection, and determination of pertinent data. The initial trials of the furnace were not successful, - 21 - so an additional course of refractory brick was placed in the front of the crucible. This additional layer of brick decreased the dimensions of the smelting zone. This decrease in vertical cross-section aided the smelting action as the current is distributed between the electrodes the same as lines of magnetic flux between two pales or in an elliptical pattern. By reducing the width of the turnace, the current assumed a more rectangular path between the electrodes and thereby increased the current density through the slag.

The bottom of the furnace was constructed in the shape of an arc and set on a horizontal framework so the furnace can be tilted for tapping. By attaching a lever to the til ting handle of the furnace" it was possible for one man to operate the furnace during the tapping pro cedure. The furnace was used for a continuous operation but could be tilted sufficiently to tap aH the mol ten material at the end of a run.

The furnace was equipped with standard water-cooled electrode holders. These were supported on a screw arrangement so it was possible to adjust the electrodes verticallyby the use of manual controls. The electrode spacing could also be changed by moving the electrodes hori zontally. This adjustment amounted to two inches; the vertical adjust ment of the electrodes was limited to seven inches. This did not affect the investigation as the electrode consumption was small, and this ver tical adjustment was sufficient for all experimental operations. The power input was regulated by the depth of immersion of the electrodes into the slag. At times the electrodes would be raised sufficiently to form arcs between the electrodes and the slag to increase the temperature of the charge surrounding the electrode. - 22-

r- \ J 1 1 \ r-- D

~ t--

f--- 1 f-- - :: t-- s:l \t') 0 .....:. •ri ("\1 [QJ .p - 1-- o.1 - t:.O .p.... Ill ~ t-- s:t H '* Q) f-- t3 s:l 1\ •ri 1 JQ 'Ô Q) v \_ Ill ~ 1> Q) H 0 • t:.O ë •rir::.. ~ ~ 0 t:.O ....s:l ~ J... Q

0 •ri .p o.1 ..r:~ 0 Cil - 23 -

Graphite electrodes were used to transmit the current to the furnaoe. The electrodes were 2 inches in diameter and 48 inches long.

At first electrodes of this length were built up from two 24-inch elec trodes which were joined by tapping the electrodes and joining them by a threaded brass plug. This procedure had one fault; the brittle graphite broke while threading the plug into the electrode and caused a loose con nection. When the joint became so loose that there was no contact except through the brass plug, the plug conducted all of the current and became very hot, eventually melting and allowing the electrode to drop into the bath. If the joint was tight and secure, no trouble was experienced, and the two electrodes functioned as favorably as one twice their length.

The main power supply is 550 volt AC current. This operates a

50 h.p. motor, direct connected to a 220 volt, 25 cycle generator. The generator is rated 30 kilowatts, three-phase; but it is connected to two transformera in such a way that single, two, or three-phase current can be obtained. The secondaries of the transformera are tapped to give llO,

55, or 27.5 volts at the mercury bus-bar tables when excited at 220 volts or half these values at llO volts. Rheostats in series with the field windings of the generator vary the excitation, so the above nominal volt ages can be varied by fractions through ranges that overlap.

Mercury tables are used to connect the secondarywindings of the transformera with the furnace. The table also has numerous con nections to the instrument board where the voltage, amperage, and power readings were taken during the investigation. As single-phase current was used, only one transformer was required. The current from the trans former is fed to bus-bars and into the furnace through heavy-duty flexible cables. .. 24-

Temperature rea.dings were taken by a platinum-platinum, 13%

rhodium thermocouple. The thermocouple was enclosed in a silica. tube which was cemented to a : inch iron pipe. To take a reading the thermo couple was im.mersed in the slag, and the difference of p otential was measured on a Brown potentiameter. The procedure was satisfactory ex cept the silica tube would break while cooling. An optical pyrometer was used to take the temperature of the slag while tapping. The slag would be partially cooled by the air so the optical reading was consis tently lower than tha t of the thermocouple.

The materials used in this investigation were Noranda copper concentrate, limestone, sand, and pyrite cinder. The analysis of these materials is in the appendix. The concentrate as received was dried and half of it was roasted, eliminating approximately half of the sulphur.

The initial trial runs used only the roasted concentrate and fluxes, but both roasted and raw concentrates were charged in the fourth and fif'th runs, giving different operating conditions which will be explained later.

Procedure The charge into the furna.ce consisted or a combination of ma.terials named in the previous paragraph. The initial charge was com posed of roasted concentrate, limestone, and sand calculated to give a slag ratio FeO:SiQ2:Ca0::40:40:10. When the raw concentrate was charged during the fourth and fifth runs, pyrite cinder was used to give the de sired slag ratio. The charge was well mixed before it was fed into the furnace. - 25 -

The starting procedure consisted of placing a layer of slag

in the bottom of the furnace. A thin iron trough filled with carbon

was placed on this slag, and the electrodes were lowered onto the carbon,

allowing the initial current to be conducted through it. The carbon soon

became incandescent, heating the slag and forming a molten bath capable

of conducting the current. When a bath of sufficient depthwas formed

to conduct the current, the mixture of concentrate and fluxes was charged

into the furnace.

It was found that, if a heavy layer of charge was maintained

over the molten bath, less heat was lost by radiation, the charge was

well preheated, and the smelting rate was increased. A deep bath of fluid

slag was built up before conductivity or temperature measurements were

taken. When the furnace was filled to capacity, part of the melt was

tapped, care being taken to leave a molten bath of sufficient depth to

conduct the current when smelting was resumed. If a shallow depth of un

melted charge was used, it would forma crust over the molten bath, seri

ously deoreasing the smelting rate from the high heat lasses of radiation.

This solid layer would usually be ~ormed by slag ~oroing its way through

the solid charge and solidifying when it came in contact with the air,

forming the solid crust. This could be minimized by keeping a heavy charge

of solid material over the molten bath, allowing the slag to come in con

tact with only the preheated charge. When the slag formed a solid layer

on top of the charge, itwas necessary to break up this layer and force

it into the melt. This crust formation caused irregular operation of

the furnace. At the end of a run, the final tapping recovered all the molten material possible from the furnace. As expected, a thin layer of - 26 - frozen charge remained on the refractory walls after tapping while other material would remain as a solid crust on top of the melt. It was pos sible to recover most of this crust from the furnace, but the material attached to the walls and the bottom of the furnace was allowed to re main and thus form part of the charge for the following run. An estimate of the material remaining in the furnace was made at the end of each run.

Readings were taken periodically, the current, voltage, and watt age being recorded. A table showing these readings during the third run is in the appendix. The conductivity measurements were taken by setting the current reading, measuring the distance between the electrodes, and measuring the voltage across the electrodes. The electrodes were then moved together a distance of two inches, the distance and the voltage again measured, keeping the amperage the same as before. A slag sample was also taken following the measurements to attempt to relate the com positions and conductivities. These data and an estimation of the depth of the slag bath made it possible to calculate the conductivity of the slag. The conductivity calculations show a close correlation to that of

Martin and Derge (14) for the same composition of slag. This result was verified by making an additional run using mostly slag and following the above procedure.

The furnace was tapped by the use of an oxygen lance, as it was necessary to burn through the slag that had solidified in the trough or the plug placed in the trough. At times the hole would be partially burned out and the remainder rammed through by an iron bar. The charge was tapped into moulds and an analysis was made of the matte and slag from each tap. These analyses are recorded in a subsequent section. The - 27 - tapping procedure required the services of at least one person basides the investigator.

Smelting Raw Concentrates

An attempt was made to roast the sulphur in the raw concentrate after it was charged in the furnace by adding excess air. A !-inch pipe was placed on each side of the furnace and connected to a low-pressure air supply. The pipes were directed down over the charge. This procedure aided the roasting, but there waa not sufficient beat present for proper roasting conditions. The aulphur eliminated was not sufficient to affect the furnace operation. In a larger furnace with more radiant heat avail able, this procedure might prove satisfactory.

1\) 1\)

()) ())

U,l U,l

q q

p:j p:j

Ef3 Ef3

~ ~

::u ::u

t-' t-'

> >

t-' t-'

~ ~

::u ::u

H H

950 950

1200 1200

1030 1030

1420 1420

1040 1040

Rate Rate

Sme1ting Sme1ting

(Kw-hr/ton) (Kw-hr/ton)

75-#= 75-#=

1.6 1.6

2.5 2.5

rode rode

2.25 2.25

tion tion

1. 1.

Elect-

Consump-

Kw. Kw.

Power Power

Input Input

Furnace Furnace

22.1 22.1

20.9 20.9

20.8 20.8

22.7 22.7

21 21

Average Average

the the

8.5 8.5

6.75 6.75

7·5 7·5

6.0 6.0

4 4

ing ing

of of

Ra.n Ra.n

Charg

After After

Hours Hours

5 5

9.0 9.0

7·25 7·25

8 8

6.5 6.5

Total Total

Çperation Çperation

II II

16 16

49 49

114·5 114·5

235 235

124 124

Tapped Tapped

Pounds Pounds

Matte Matte

TABLE TABLE

Functional Functional

92 92

41 41

76 76

46 46

161 161

and and

Slag Slag

Tapped Tapped

Pounds Pounds

=If =If

Q) Q)

J.. J..

s:l s:l

Q)'\j Q)'\j

+' +'

~ê3 ~ê3

14 14

26.8 26.8

Materials Materials

] ]

Cl) Cl)

1·7 1·7 2.61/= 2.61/=

1·9 1·9

6.0 6.0

Charging Charging

Q) Q)

8 8

!Il !Il

.s .s

....1 ....1

.!i .!i

6.0:/1= 6.0:/1=

17.1 17.1

13.3 13.3

29.5 29.5

18.0 18.0

Materials Materials

=If =If

s:l s:l

Q) Q)

0 0 s:l s:l

.P .P

o o

itS itS

il:; il:;

p:;o p:;o

180 180

264 264

s:l s:l

Q) Q)

s:l s:l

0 0

.P .P

37 37

49 49

Q) Q)

~o ~o

til til

+' +' '0 '0

p:;o p:;o

100/f 100/f

301 301

212 212

Total Total

Charge Charge

326.9 326.9

364.2 364.2

263.1 263.1

108.6 108.6

235·1 235·1

# #

3 3

5 5

1 1

2 2

4 4 Run Run TABLEIII

Percentage Composition or S1ags and Mattes Produced

Slag Per Cent Composition Matte Per Cent Composition

Run 4f: Tap f FeO CaO Si02 Cu ZnO Cu Fe s Zn 1 1 31.9 5.0 43.1 1.15 - 46.2 20.8 22.9 2 1 39·5 5.87 30.09 2.35 - 45.91 19-78 18.97 2 4D.4 6.48 28.35 2.75 - No samp1e ta.ken 44.2 6.05 25.38 3.68 53.17 16.73 21.43 3 - • 1\) 3 1 49.2 7.12 24.11 1.43 - No matte tapped 'Û 2 51.5 5·67 19.54 5.12 - 53.61 16.71 21.27 4 1 35.06 10.82 29.94 5.0 4.25 29.68 36.24 23.77 2 30-09 10.42 31.85 5·1 4.75 26.04 37.68 25.50 5 1 31.58 J.2.53 32.75 2.50 3·98 26.87 38.34 22.76 4.16 2 26.85 13.92 36.68 1.60 3·41 25.14 38.74 23.21 4-35 3 No slag tapped 26.55 ;8.44 25.45 4·37 - 30-

TABLE IV

Precious Metal Recovery in Mattes

Gold Silver Run # oz/ton oz/ton

1 .224 13.786 2 .128 5-976 3 -378 14.424 4 .228 7.480 Slag Sample 3 .004 0.382

Theoretical Heat Balance for Furnace

Based on data from the third trial run given in the appendix.

Kgm-Cal Per Cent

Input

E1ectrical Heat Energy 108,000 96.0 Electrical Heat Energy by reaction of carbon to form CO 5,460 4.0

Total 113.460 100.0 Heat Content of Brick 20,000

Net Input 93,460 Output

Heat Content of Matte 12, 7r:IJ 13-7 Heat Content of Slag 14,800 15.8 Heat absorbed in the formation of S(),2 4,560 4-9 Heat loss by conduction and radiation 15,000 16.1 Heat loss through water cooling system, mise. 46,350 49-5

Total 93,460 100.0

Calculated Furnace efficiency - 34.4%

The thermal data were taken from Butts(5) and Trinks(4). - 31 -

DISCUSSION OF RESULTS

The data show that it is entirely feasible to smelt copper concentrates using an electric furnace. The experimental furnace~ al though small in size, functioned properly throughout the investigation and no major difficulties were encountered. In a commercial operation, the furnace size will depend on the amount of material to be processed in a day and also its nature and composition. As pointed out previously, in many regions in Canada electricity is the most economical source of heat energy available.

If an electric furnace can be operated in a region where there are no smelters, a high grade matte produced would undoubtedly afford an econamical recovery of the copper in the ores. The freight charge on the copper and precious metal content of a good grade of matte will be much smaller than the freight on the original material before smelting.

The recommended type of electric furnace to use is a resistance furnace, the heat produced by the resistance of the slag to the current passing through it. The furnace should be designed for a continuous opera tion, the raw material charged constantly or intermittently as often as required to maintain a good layer of unfused charge over the bath, and the matte and slag tapped constantly into an outside settler. By using a con tinuous operation and a larger furnace, the thermal efficiency and the smelting rate will be greatly increased over the values found in this in vestigation. Under these conditions, the furnace will require little attention or maintenance; the work consisting mostly of supervising the charging and tapping. The working conditions are not objectionable in anyway,which will tend to ease the labor conditions. There will be a - 32 - minimum of flue dust or gases from the furnace, and the external heat oonducted through the walls will not be excessive. The furnace would be operated by automatic controls to minimize the amount of labor neces sary and to regulate the furnace properly.

The products of the tests show that the recovery of copper and precious metals in the matte is wholly adequate. The peroentage of cop per in the slag was high because the melt was poured into small moulds which chilled and froze the melt almost instantly, giving no time for a separation of the matte and slag. The grade of matte ranged from 26 to

51 per cent during the investigation. The low grade matte occurred when the unroasted concentrate was charged. The electric furnace would smelt high sulphide ores, but the furnace operation would be more complex than when ores of lower sulphur content were charged. If the oost of roasting is not prohibitive, roasting of high sulphide ores would be advised al though not campletely necessary. These ores would require additional fluxes to keep the slag bath uniform; the sulphur would take most of the iron of the charge into the matte, neoessitating the addition of iron into the flux unless converting is practised and the iron in the matte is re turned to the smelting furnace in the converter slag. A commercial fur nace with a closed roof might have sufficient radiant heat to roast pro parly a high sulphide ore using low pressure air over the charge.

The smelting rate of the furnace will be limited by the size of the furnace. The power consumption of the furnaee used in the investiga tion was high but this is to be expected in a small pilot furnace. A factor that requires much consideration in the use of the electric furnace is the high cost of electrical energy. This high cost is such that - 33-

electric furnace operation is econamically prohibitive under most condi

tions; but the advantages of electric smelting such as its adaptability,

good working conditions, and excellent recovery of copper and precious

metals are such that serious consideration should be given to this type

of smelting in many areas. The increasing electrical power supply in

this country will undoubtedly provide an electrical rate that will make

the electric smelting of copper concentrates definitely advantageous in

the future.

Design of Small Commercial Type Furnace

A furnace was designed from the data taken during the investi

gation for a small commercial operation. This furnace is shown in Figures

V and V(a). It is designed to use 1000 kilowatts of power and will treat 40 tons of charge daily, using 600 kilowatt-hours per ton of ore. The

heat will be generated by three-phase current passing through three pairs

of 8-inch graphite electrodes using a delta connection. The electrodes will be placed in a line, each pair of electrodes connected to a separate

phase of current. The electrodes will be controlled automatioally to keep

a constant power input. The inside dimensions of this furnaoe are 12 by 2 by 3 feet in

the smelting zone, giving a capacity of 72 cubic feet. The height of the

furnaoe is increaeed 18 inches to conserve heat and facilitate charging.

The refractories used are the same as those in the experimental furnace.

The furnace walls have a 9-inch course of refractory brick backed by a

4i;inch course of insulating brick. The bottom of the furnace is built by using a l3à-inch course of refractory brick baoked by a 4à-inch course -~- of insulating brick. A one-inch layer of crushed insulating brick is placed between the insulatingbrick and the shell of the furnace to al law for the thermal expansion of the brick. The bottom of the furnace has one side sloping towards the center where a matte trap is constructed for constant tapping. This sloping trough is 7l inches wide to collect the matte as it is fonned so the current will not short-circuit through it. The walls of the furnace are increased in height by using a ~-inch course of insulating brick. An arched roof will cover the furnace and will contain charging doors as shown in the schematic drawing, Figure V(a).

The furnace is stationary, and the bottom is air cooled. The bottom of the furnace will set on a ~inch steel plate supported by steel angles and welded or riveted in place. The angles will set on the concrete forms. The supports for the walls may be I-be~s firmly imbedded in the floor with iron rods connecting them with sufficient allowance for the thermal expansion of the brick.

With a furnace of these dimensions, it would be possible to have the electrical apparatus,including the electrode holders and temperature measuring instruments, on one side of the furnace, leaving the other side clear for charging and tapping. Standard water-cooled electrode holders can be used with automatic power controls. This is one type of furnace that might be used for a small commercial operation. The thermal efficiency of the furnace is calculated to be approximately 53 per cent.

The theoretical heat balance of this furnace, using calculations based on the charge used during the investigation and thermal data from Butts(5) and

Trinks(4) is as follows: - 35-

Theoretical Heat Balance

B. t.u. Per Cent

Input

Electrical Heat Energy 81,700,000 Heat generated by reaction 261,000 of carbon to form CO

Total 81,961,000 100.0

Output

Heat Content of Matte 17,200,000 21.00% Heat Content of Slag 20,000,000 24.4 Heat absorbed in fonnation of SD2 6,040,000 7-35 Heat loss through furnace walls 2,660,000 3-25 Mise. hea.t loss 36,061,000 44.00

Total 81,961,000 100.0 ~

Trap

>-1

~Matte

r--s/'

Holder

Electrode

Electric

Type

---A'

-1--+--

Furnace

Cmrunercial

Fig.

Smelting

Drawing

Il•

!...W.-!.

---==~

171''

Sehematic

-1

---+-

A Section A-A'

Charging Doors

\..N-J Section B-B'

Fig. V(a) Sectional Views of Commercial Type Furnace - 38 -

SUMMARY

This investigation has proven that the electric furnace is capable of smelting copper concentrates. The furnace can be adapted to practica1ly any size operation, being particularly suited to small scale operations which cannot be operated econo.mically under present conditions.

The results of the investigation show that the electric fur nace has definite advantages; it can produce a desirable grade of matte capable of recovering practically all of the precious metals, it will operate under a minimum of supervision, and the working conditions will be very satisfactory. A heat balance of the furnace used for the in vestigation shows the efficiency to be adequate. The thermal efficiency will increase in a large furnace.

An electric furnace was designed from the data taken during the investigation to show the type of furnace that may be used in a small ecale commercial operation. The cast of electric energy is one consideration which must be carefully investigated before installing an electric furnace. The high cost of electrical energy is the main dis advantage of using this type of furnace. Its operational procedure is otherwise very favorable for smelting copper ores and concentrates. - 39 -

APPENDIX

Analysis of Material Used in the Investigation:

(a) Noranda Copper Concentrate Copper (Cu) 21.66% Sulphur (S) 29.33% Iron (Fe) 26.50% (Total iron expressed as Fe) Zinc (Zn) 4-46% Alumina (Al2o3) .21% Lime (CaO) .10%

(b) Roasted Ore (two samples) #1 Iron (Fe) 1/2. Iron (Fe) 29.51% Copper (Cu) Copper (Cu) 21.61%

Sulphur (s) Sulphur (s) 14.12%

( c) Lime stone

Lime (CaO) 54.16% Silica (Si02) 2.09% (d) Silioa Sand

Silica {Si~) 97.04% Mise. oxides of negligible proportions

(e) Pyrite Cinder Iron (Fe) 66.32%

Silioa (Si02 ) 1.43% All analyses of these materials and the mattes and slags were made by the ·

Province of Quebec Department of Mines under ~~e direction of Dr. Maurice

Archambault. - 40-

TABLE V Table of Data Taken During Run #3

Amps. at Excitation Transformer Input Ti.me Mo tor Voltage Taps Voltage Amps. Volts Watts

0915 40 240 110 256 97 25,600 0935 43 205 110 352 69 27,200 1000 40 200 55 800 36 24,000 1015 39 190 55 640 35 22,400 1030 34 270 27.5 640 29 19,200 1050 30·5 255 27.5 575 28.5 16,000 1100 30 250 27.5 505 1125 40 205 55 575 38·5 22,400 1145 40 180 55 750 28 20,800 1200 39 26o 27.5 830 27·5 22,400 1220 40 250 27.5 895 27 24,000 1240 40 240 27 ·5 1020 25 24,800 1300 34 205 27.5 945 21 22,400 1320 36 210 27-5 970 21.5 21,600 1410 36 245 27.5 795 26.5 20,800 1440 37 245 27.5 825 26.5 21,6o0 1455 28 16o 27.5 890 17 12,800 1510 32 250 27.5 645 27·5 17,600 ' 1525 32 250 27·5 670 27·5 18,400 1550 33 250 27.5 670 27 18.,400 1610 30 185 27.5 810 19 15,200 1650 end of run - 41-

Fig. VI

Electric furnace used during the investigation showing the auxiliary equipment.

Fig. VII XlOO

Slag showing included matte, composition FeO 35.06%, Si02 29.94%, CaO 10.82%, Cu 5.0%. Fig. VI II -r XlOO

Matte containing 53.17% copper, showing homogeneous solid solution and free copper.

Fig. IX XlOO Matte containing 36.24% copper, showing the eutecti ferous structure. - 43-

BIBLIOGRAPHY

Books

1. Bray., J.L. Non-Ferrous Production Metallur~y, second edition. New York: John Wiley and Sons, 1940

2. Newton, Wilson. Metallurgy of Copper, New York: John Wiley and Sons, 1942

3. Stansfield, A. The Electrical Furnace, New York: McGraw-Hill Book Company, 1914

4. Trinks, W. Industrial Furnaces, Vol. I & II, New York: John Wiley and Sons, 1925

5. Butts, A. Metallurgical Problems, New York: McGraw-Hill Book Com pany, 1943

Articles

6. "Analysis of Copper and Nickel Slags and Mattes." Metallurgia, Vol. 37 (January, 1948). PP• 153-156

7. Beckman, J.W. "The Plating of Iron from a Bath of cao-Fe203." Transactions of American Electrochemica1 Societl, Vol. 19 (1911). PP• 171-181

8. Bowen, Shairer, and Posnjak. "The Ternary System Ca0"FeO•Si02 ." American Journal of Science, Vol. 26 (1933). pp. 273-297 9· Cambell, A. "Experiences in Pouring Copper with High Conductivity." Foundrx. Vol. 71 (July., 194)). pp. 112 10. "Copper Ferrite from Slag Reaction." AIME Transactions, Vol. 188 (1949). PP• 158 11. Helbrugge and Endell. Archiv Eisenhuttenw, Vol. 14 (1941). pp. 307-315

12. Herasyrnenko, P. "Ionization Theory of Slags." Transactions of Faraday Societx, Vol. 34 (1938). pp. 1245-1254 13. Herasymenko and Speight. "Ionie Theory of Slag-Metal Equilibria." Journal of Iron and Steel Institute, Vol. 166 (November, 1950). PP• 169-183 -~-

BIBLIOGRAPHY Cont.

14. Martin and Derge. "Electrica1 Conductivity of Molten B1ast Furnace S1ags." Iron Age, Vol. 151 (February 25, 1943). PP• 59-60

15. McCaffery, R.S. "Research on Blast Furnace Slags." AIME Trans actions, Vol. 100 (1932). pp. 64-121 16. "Modern Copper Smelting." Metals Technology. Vol. 11 (February, 1944) 17. Newhal1, H.S. "Experimental Electric Arc Furnace Smelting." Transactions of Electrochemical Society, Vol. 88 (1945)

18. Olsen, R.P. 11 Relationship between Conductivity and Composition of Molten PbSi04 S1ags." Journal of Metals, (December, 1949). pp. 984-986

19. Osborn, C.J. 11 The Graphical Representation of Metallurgical Equi libria." Journal of Metals, Vol. 188 (March, 1950). pp. 600-607 (Free Energy Drawings)

20. Post, C.B. "Slag Materia1s and Reaction in Basic Arc Furnace." Journal of Metals, (November, 1950). PP• 1313-1315 21. Richardson, F.D. "Theoretical Aspects of the Structure of Slags and Its Re1ationship to S1ag-Metal Equilibria." Discussions of the Faraday Society, No. 4 (1948). PP• 244-257

22. 11 Thermodynamics of Copper Smelting." AIME Transactions, Vol. 188 (1949). P• 158 23. Wejnarth, A. 11 Current Conducting Properties of Slags in Electric Furnaces." Transactions of Electrochemical Society, Vol. 65-66 (April, 1934). PP• 319-329 24. Ke11ey, K.K. u.s. Bureau of Mines Bulletin, No. 371 (1934) 25. Kelley, K.K. U.S. Bureau of Mines Bulletin, No. 383 (1935) 26. Kelley, K.K. u.s. Bureau of Mines Bulletin, No. 393 (1936)

27. Kelley, K.K. u.s. Bureau of Mines Bulletin, No. 406 (1937) 28. Kelley, K.K. u.s. Bureau of Mines Bulletin, No. 434 (1941/1948)